VP-1-16 undergoes O-demethylation to generate active intermediates that bind to proteins and DNA. The O-demethylation is P450-dependent. Peroxidases, such as horseradish or prostaglandin synthetase, also activate VP-16 and VM-26 to their O-Quinone derivatives and catalyze the binding of reactive intermediates to DNA. We have shown that both the dihydroxy and O-Quinone derivatives are cytotoxic and induce topo-II- dependent cleavage. The binding sites on the topo-II-DNA complex for these O-demethylated drugs are similar to those of the parent compound. We have also shown that the dihydroxy VP-16 binds metal ions (iron and copper). These metal ion complexes are redox-active and induce DNA strand scission in an oxygen-dependent pathway. Thus, enzymatic activation to reactive intermediates is important in the biological activities of VP-16 and VX-26.